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  <TITLE>Apache module mod_expires</TITLE>
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  <H1>Module mod_expires</H1>
  <P>
  This module is contained in the <CODE>mod_expires.c</CODE> file, and
  is <STRONG>not</STRONG> compiled in by default.  It provides for the
  generation of <CODE>Expires</CODE> headers according to user-specified
  criteria.
  </P>
  <H2>Summary</H2>
  <P>
  This module controls the setting of the <CODE>Expires</CODE> HTTP
  header in server responses.  The expiration date can set to be
  relative to either the time the source file was last modified, or to
  the time of the client access.
  </P>
  <P>
  The <CODE>Expires</CODE> HTTP header is an instruction to the client
  about the document's validity and persistence.  If cached, the document
  may be fetched from the cache rather than from the source until this
  time has passed.  After that, the cache copy is considered
  &quot;expired&quot; and invalid, and a new copy must be obtained from
  the source.
  </P>
  <H2>Directives</H2>
  <P>
  <MENU>
   <LI><A
        HREF="#expiresactive"
       >ExpiresActive</A>
   </LI>
   <LI><A
        HREF="#expiresbytype"
       >ExpiresByType</A>
   </LI>
   <LI><A
        HREF="#expiresdefault"
       >ExpiresDefault</A>
   </LI>
  </MENU>
  <HR>
  <A NAME="expiresactive">
   <H2>ExpiresActive directive</H2>
  </A>
  <!--%plaintext &lt;?INDEX {\tt ExpiresActive} directive&gt; -->
  <P>
  <STRONG>Syntax:</STRONG> ExpiresActive <EM>boolean</EM>
  <BR>
  <STRONG>Context:</STRONG> server config, virtual host, directory, .htaccess
  <br>
  <STRONG>Override:</STRONG> Indexes
  <br>
  <STRONG>Status:</STRONG> Extension
  <br>
  <STRONG>Module:</STRONG> mod_expires
  </P>
  <P>
  This directive enables or disables the generation of the
  <CODE>Expires</CODE> header for the document realm in question.  (That
  is, if found in an <CODE>.htaccess</CODE> file, for instance, it
  applies only to documents generated from that directory.)  If set to
  <EM><CODE>Off</CODE></EM>, no <CODE>Expires</CODE> header will be
  generated for any document in the realm (unless overridden at a lower
  level, such as an <CODE>.htaccess</CODE> file overriding a server
  config file).  If set to <EM><CODE>On</CODE></EM>, the header will be
  added to served documents according to the criteria defined by the
  <A
   HREF="#expiresbytype"
  >ExpiresByType</A>
  and
  <A
   HREF="#expiresdefault"
  >ExpiresDefault</A>
  directives (<EM>q.v.</EM>).
  </P>
  <P>
  Note that this directive does not guarantee that an
  <CODE>Expires</CODE> header will be generated.  If the criteria aren't
  met, no header will be sent, and the effect will be as though this
  directive wasn't even specified.
  </P>
  <HR>
  <A NAME="expiresbytype">
   <H2>ExpiresByType directive</H2>
  </A>
  <!--%plaintext &lt;?INDEX {\tt ExpiresByType} directive&gt; -->
  <P>
  <STRONG>Syntax:</STRONG> ExpiresByType <EM>mime-type &lt;code&gt;seconds</EM>
  <BR>
  <STRONG>Context:</STRONG> server config, virtual host, directory, .htaccess
  <br>
  <STRONG>Override:</STRONG> Indexes
  <br>
  <STRONG>Status:</STRONG> Extension
  <br>
  <STRONG>Module:</STRONG> mod_expires
  </P>
  <P>
  This directive defines the value of the <CODE>Expires</CODE> header
  generated for documents of the specified type (<EM>e.g.</EM>,
  <CODE>text/html</CODE>).  The second argument sets the number of
  seconds that will be added to a base time to construct the expiration
  date.
  </P>
  <P>
  The base time is either the last modification time of the file, or the
  time of the client's access to the document.  Which should be used is
  specified by the <CODE><EM>&lt;code&gt;</EM></CODE> field;
  <STRONG>M</STRONG> means that the file's last modification time should
  be used as the base time, and <STRONG>A</STRONG> means the client's
  access time should be used.
  </P>
  <P>
  The difference in effect is subtle.  If <EM>M</EM> is used, all current
  copies of the document in all caches will expire at the same time,
  which can be good for something like a weekly notice that's always
  found at the same URL.  If <EM>A</EM> is used, the date of expiration
  is different for each client; this can be good for image files that
  don't change very often, particularly for a set of related documents
  that all refer to the same images (<EM>i.e.</EM>, the images will be
  accessed repeatedly within a relatively short timespan).
  </P>
  <P>
  <STRONG>Example:</STRONG>
  </P>
  <P>
  <PRE>
   ExpiresActive On                  # enable expirations
   ExpiresByType image/gif A2592000  # expire GIF images after a month
                                     #  in the client's cache
   ExpiresByType text/html M604800   # HTML documents are good for a
                                     #  week from the time they were
                                     #  changed, period
  </PRE>
  </P>
  <P>
  Note that this directive only has effect if <CODE>ExpiresActive
  On</CODE> has been specified.  It overrides, for the specified MIME
  type <EM>only</EM>, any expiration date set by the
  <A
   HREF="#expiresdefault"
  >ExpiresDefault</A>
  directive.
  </P>
  <HR>
  <A NAME="expiresdefault">
   <H2>ExpiresDefault directive</H2>
  </A>
  <!--%plaintext &lt;?INDEX {\tt ExpiresDefault} directive&gt; -->
  <P>
  <STRONG>Syntax:</STRONG> ExpiresDefault <EM>&lt;code&gt;seconds</EM>
  <BR>
  <STRONG>Context:</STRONG> server config, virtual host, directory, .htaccess
  <br>
  <STRONG>Override:</STRONG> Indexes
  <br>
  <STRONG>Status:</STRONG> Extension
  <br>
  <STRONG>Module:</STRONG> mod_expires
  </P>
  <P>
  This directive sets the default algorithm for calculating the
  expiration time for all documents in the affected realm.  It can be
  overridden on a type-by-type basis by the
  <A
   HREF="#expiresbytype"
  >ExpiresByType</A>
  directive.  See the description of that directive for details about
  the syntax of the argument.
  </P>
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